Hitherto, certain types of 4-oxoquinoline-3-carboxylic acid compounds having antibacterial activity have been known. For example, JP-A-58074664 (EP-A-78362) describes 1-cyclopropyl-6-fluoro-1,4-dihydro-7-(1-piperazinyl)-4-oxoquinoline-3-carb oxylic acid of the formula ##STR2## (particularly in Example 3, which is called cyprofloxacin or "Ba-2" hereinafter), and similar compounds are also disclosed in JP-A-59163369 (EP-A-117473), JP-A-59130880 (EP-A-113093) and JP-A-59212474 (EP-A-126355). Of these compounds, cyprofloxacin has been reported to have excellent properties and promisingly it will be available as commercial product in near future.